For example, as disclosed in U.S. Pat. No. 5,491,631 (JP 5-172705A), an abnormality diagnosis system is proposed to diagnose the presence or absence of an abnormality in an actuator of an internal combustion engine while communicating with an electronic control unit of an internal combustion engine. Specifically, in this abnormality diagnosis system, a valve angle command is issued to an exhaust gas recirculation (EGR) valve for controlling the cross-sectional area of an exhaust recirculation passage (EGR passage) for recirculating exhaust gas from an exhaust system of the internal combustion engine to an intake system, and it is determined that the EGR valve is abnormal when the EGR valve is not opened after a specified period. Thus, the presence or absence of an abnormality in the operation of angle the EGR valve can be diagnosed.
In this diagnostic method, an operating signal is outputted to an actuator of an object to be diagnosed, and then at a specified moment after a specified period the presence or absence of an abnormality in the actuator is diagnosed based on the state of the actuator. Thus, the presence or absence of an abnormality in the actuator cannot be determined in a transition period from an operating command is issued to the actuator until the actuator finishes a desired operation. For this reason, an abnormality that the change of the actuator in the transition period until the actuator finishes the desired operation, that is, an abnormality in response characteristic cannot be appropriately detected.